A variety of optical-electrical systems exist for projecting images or information on a display screen sized tens to hundreds of inches, from a microdisplay imager in a size of sub inch. Such a microdisplay imager consists of a planar array of light modulation micro-scale pixels fabricated on a silicon substrate. The optical reflectance of each pixel is electrically modulated in situ by an underlying CMOS-based circuitry fabricated on the same silicon substrate. Most common of such reflective microdisplay imagers are liquid crystal on silicon (LCOS), deformable mirror device (DMD) and galvanic light valve (GLV).
A variety of optical engine and projection lens assembles are employed and assembled, for adequately inducing light ray to such a reflective microdisplay imager and then optically projecting the image formed in differentiated gray scales of reflected light on the microdisplay imager to a display screen. Such an optical system, often called optical engine assembly, at least consists of a light source, a reflective microdisplay imager, a projection lens (or lens), and last but not least, an optical device, often called engine core, inducing the illumination light from the light source to the reflective microdisplay imager, in which a reflective microdisplay imager and the projection lens are mounted in parallel on the opposite sides of the engine core. Fixed in a tilted angle with the microdisplay imager and projection lens, the engine core consists of at least one optical surface, receiving and deflecting portion of illumination light from the light source towards the microdisplay imager. Portion of spatially modulated light reflected from the source light by the imager passes through the engine core and the projection lens and thus, is projected on a display screen forming an enlarged image. Prior art shown in the patents, such as U.S. Pat. No. 5,552,922, U.S. Pat. No. 5,604,624, U.S. Pat. No. 6,461,000, U.S. Pat. No. 6,490,087, EP 2000/0830425, and US 20080012805, by Magarill, Lambertini and Duncan well exemplifies the basic optical framework of such an optical engine assembly and projection system. More sophisticated projection systems employ an engine core combining two or more of such optical surfaces in crossing configuration for inducing light to multiple microdisplay imagers and constructing the images of different color or light spectrums to a single projection display.
Such a projection system is miniaturized to a “micro” or “pico” system, with both optical engine and imager shrunk proportionally, for various portable and mobile handheld applications. In such applications, power consumption of a micro or pico projection system is often of serious concern. Meanwhile, optical efficiency of such a projection system is far from 100% and so is the net energy efficiency from electrically powered light source to projection illumination out of projection lens. Diffraction and deflection away from the main illumination beam paths by various surfaces, as well as light scattering by transparent medium, such as air, in such a projection system are among the main causes to such loss in optical efficiency and thus electrical energy. It is highly desirable to collect and convert such unused illumination in order to recover part, if not most of energy loss, preferably in situ within the projection system and to re-store it into a built-in energy storage device, particularly into a rechargeable battery, and to reuse recovered energy for partially powering the light source and/or microdisplay imager, as well as other electrically powered devices in such a handheld device.
As photovoltaic device technology advances, more than 20% of photonic-electrical energy conversion efficiency could be achieved. Such micro or pico projection systems often require illumination in a fairly high intensity from its light source, but the overall optical efficiency is in low percentages. Loss due to light reflection and deflection by various surfaces as well as light scattering by transparent material enclosed in their optical system contributes substantially to efficiency reduction. Thus, potential and need for recovering such energy loss due to unavoidable optical artifacts and converting portion of unused illumination to reusable photogenerated charge is considerable for extending service time of the built-in rechargeable battery.
However, in a portable reflective projection system employing a single-panel LCOS imager as a the reflective microdisplay imager 20 and a simple optical engine as shown in FIG. 1, considerable loss in optical efficiency still results from polarization of illumination light by a transmissive polarizing film 11 providing polarized source light 12 towards a light-redirecting mirror 14. Second, a low cost transmissive polarizing film 11 is often made of polymeric materials so that its thermal stability and radiation aging are of concern under various adverse application environments. Further advance of the prior art from the system shown in FIG. 1 is made by employing a reflective transmissive polarizing film 11a, replacing both the transmissive polarizing film 11 and the light-redirecting mirror 14, as shown in FIG. 2. Even though majority of the P-component of incident light is reflected for illumination eventually to the LCOS imager, the S-component is deflected and absorbed as waste energy by the reflective polarizing film 11a. It is highly desired that such loss of energy could be partially recovered while the optical engine system is configured to have sufficient temperature stability and mechanical compactness at low cost without additional components. Such reflective polarizing film or panel can be fabricated as shown in U.S. Pat. No. 7,158,302 by Yu et al, or U.S. Pat. No. 6,970,213 by Kawahara, et al.